Adjustable sliding brush transformer and method of producing same



Nov. 25, 1969 J HELBERG ET AL 3,480,897

ADJUSTABLE SLIDING BRUSH TRANSFORMER AND METHOD OF PRODUCING SAME FiledSept. 5', 1967 2 Sheets-Sheet 1 ii/z ills g M INVENTORS.

flttorwe Nov. 25, 1969 H. J. HELBERG ET 3,430,897

ADJUSTABLE SLIDING BRUSH TRANSFORMER AND METHOD OF PRODUCING SAME FiledSept. 5, 1967 2 Sheets-Sheet E ,-,ZZ;Z;Z?Z" Z a g 7 7 93 f w ,1:

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Ind., assignors to General Electric Company, a corporation of New YorkFiled Sept. 5, 1967, Ser. No. 665,402 Int. Cl. H01f 29/06 U.S. Cl.336-149 6 Claims ABSTRACT OF THE DISCLOSURE An adjustable sliding brushtransformer having a backing strip of interstitial or permeableinsulating material sandwiched between the contact surface portion ofthe winding and the core insulating member. The electrical contactsurface portion of the winding is bonded to the core insulating memberby a cured thermosetting resin carried in the backing strip. The backingstrip is disposed along the outer periphery of the core insulatingmember adjacent to the contact surface portion of the winding. In theprocess of manufacture the coil turns are wound over the backing stripand around the insulating member with the thermosetting resin impregnantbeing in a semicured and tacky condition. Preferably, the coil turnportions of the winding which form the contact surface portion areembedded in the backing strip and when cured, the thermosetting resinfirmly bonds the contact surface portion of the winding to the coreinsulating member.

BACKGROUND OF THE INVENTION This invention relates generally toadjustable sliding brush transformers, and more particularly toadjustable sliding brush transformers having improved brush tracks andmethods of producing same.

Variable transformers of the sliding brush variety typically have anautotransformer coil wound as a toroid upon an annular magnetic corewith a surface portion of the coil forming a brush track. A brushholder, carrying one or more electrical brushes is arranged to permitthe brushes to slide across and make electrical contact with the brushtrack. The output voltage of the transformer is varied by changing theposition of the brushes with respectto the brush track, and hence it isfrequently necessary to slide the brushes along the track.

One source of trouble with sliding brush transformers is in theconstruction of the brush track. It often happens that one or moreindividual turns of the coil forming the brush track is either depressedbelow or raised above the plane of the remainder of the brush track,resulting in a rough, uneven and/or irregular track. Under thiscondition, the brushes may catch at the uneven areas and either breakoff or be pulled from their brush holder. Further, an uneven track maycause the brushes to rotate in their brush holder and, if the brushesare then carrying load current, arcing may occur if a brush momentarilyloses contact with the track. This may result in damage to the brushes,the brush holder or the conductor turns. At the very least, an unevenbrush track causes hot spots, uneven and elevated temperatures andpossible burning andex'cessive wearing of the brushes. It should beunderstood therefore that the rejection rate of the variable or slidingbrush transformers due to brush track defects is quite high.

One cause of uneven or irregular brush tracks in sliding brushtransformers is a lack of adequate backing or support behind the coil orwinding turns at the track area. Without a firm and uniform backing atthis area, spaces may occur behind the conductor turns, permitting aturn to fall below the track level, or conversely forcing an adjacentturn above the level of the track. It has been suggested to incorporatea mat or blanket behind the conductor turns and to dip the assembly invarnish or the like to provide backing support for the brush track. Thisapproach has not been successful since the varnish often does not fullypenetrate into the mat or blanket and hence some of the turns still areable to move away from the plane or surface of the track. Further, evenwhen turns were pounded or depressed into the mat or blanket, thefinished track surface was often irregular since the resilient coilturns would spring back away from the mat and leave spaces or gapstherebehind.

Another prior art approach involves the application of an adhesivecoating on the core at the track area between the core and the coil.This technique is not satis factory since during fabrication, theassembly is messy and hence difficult to handle after the coating layeris applied. Also, when the winding is placed on the core, there is atendency for the coating layer to extrude upwardly between adjacentturns of the winding since it is in a relatively fluid state at thistime. Certain materials, such as polyester resins, cannot be used asthey would contaminate the carbon brushes upon contact. Further if apolyester resin were used and exposed above the track surface, it wouldbecome soft and flow when subjected to the normal temperature of thetransformer.

Accordingly, it is a general object of the present invention to providean improved sliding brush type of variable transformer which has astably bonded brush track and a method manufacturing such a variabletransformer.

It is a more specific object of the present invention to provide animproved variable transformer and method of producing the same whereinthe individual coil turns comprising the brush track are firmly held inplace to form an improved and stable brush track.

It is another object of the present invention to provide an improvedsliding brush transformer and method of producing same wherein arcingalong the brush track due to displaced turns is effectively minimized.

SUMMARY OF THE INVENTION In carrying out the objects of this inventionin one form, we provide an improved variable transformer of the slidingbrush type that includes an annular core of magnetic material with ashell of insulating material disposed on the core. A carrier or backingstrip of interstitial or permeable insulating material is disposedcircumferentially around at least a portion of the shell, with athermosetting resin in a semicured or B state in the carrier strip. Awinding of successively disposed adjacent coil turns is disposed on theshell with portions of a majority of the coil turns being bonded theretoby the backing strip and, if desired, being partially embedded in thecarrier strip. Segments of these coil turn portions are removed andprovide a flat electrical contact surface or brush track. In addition, abrush holder carrying one or more electrical brushes is disposed forsliding electrical contact with the fiat electrical contact surface orbrush track.

In accordance with another aspect of the present invention, in one form,we provide an improved method of fabricating or producing an adjustablesliding brush transformer that comprises assembling an annular magneticcore in a shell, wrapping a backing strip having a thermosetting resinin a semicured state impregnated therein and winding or placing aplurality of adjacent turns on the shell to form a toroidal windingthereon with portions of the turns overlying the backing strip. At thistime, a compressive force is applied to the turn portions in order toinsure that these turn portions are straight and in contact with thebacking strip, and, if desired, to push the turn portions radiallyinwardly and at least partially embed them into the backing strip. Theentire assembly is then cured at an elevated temperature to harden thepolyester resin and thereby structurally bond the turn portions to thebacking strip and the backing strip to the outer peripheral surface ofthe shell. Outer segments of the turn portions are then removed therebyleaving a generally flat surface on each outer turn portion, with theseflat surfaces being at substantially the same distance from the axialcenter of the core in order to provide a flat, uniform and regular brushtrack for contact with the electrical brushes subsequently assembled ina brush holder and slidably mounted for contact with the 'brush track.

A primary advantage of the briefly described transformer and method offabricating the same is that the backing provided for the coil turns atthe brush track area rigidly secures the coil turns in place to providea stable brush track. In this regard, a more solid backing is notnecessary, since the coil turns, when compressed into the backing strip,will not spring back. This is due to the tacky or adhesive nature of thebacking strip, with its impregnated polyester resin in a semicuredstate. Thus, the tacky or adhesive backing strip facilitates thefabrication of the transformer as it retains the compressed coil turnstightly against the transformer shell when they are pressedthereagainst. Furthermore, the backing strip acts as a carrier for thepolyester resin, substantially preventing it from being squeezed outbetween the coil turns when the coil turns are compressed, and once theployester resin is cured, the coil turns are structurally bonded inplace, thereby preventing any of the turns from becoming loose andspoiling the uniformity of the brush track.

The subject matter which we regard as our invention is set forth in theappended claims. The invention itself, however, together with furtherobjects and advantages thereof may be best understood by referring tothe following description taken in connection with the accompanyingdrawings.

Brief description of the drawings FIGURE 1 is a perspective view of anadjustable sliding brush transformer embodying one form of the presentinvention;

FIGURE 2 is an enlarged partial sectional view taken substantially onthe plane of the line 2-2 of FIG- URE 1;

FIGURE 3 is an exploded perspective view showing a first step in theprocess of assembling certain components of the transformer assembly inaccordance with the method of fabricating the transformer illustrated inFIG- URE 1;

FIGURE 4 is a front elevational view showing another step in the processof fabricating the transformer illustrated in FIGURE 1 wherein the turnportions are pressed 7 Description of the preferred embodiment Referringnow to the drawings, the improved adjustable transformer, generallydenoted by reference numeral 10, comprises a core 12 of magneticmaterial substantially encased in an insulative shell denoted generallyby reference numeral 14. The shell 14 includes upper and lowersubstantially identical shell halves 16 and 18 respectively which are ofa suitable insulating material such as phenolic resin or the like. Woundaround the core 12 and the shell 14 of the transformer is a winding 20of insulated wire. A portion of the surface of the winding has theinsulation and some copper removed after the turns are flattened in amanner to be described hereinafter to form a generally flat annularbrush track generally denoted by reference numeral 22. The core and coilassembly of the transformer 10 is bonded within a wall 25 on a baseplate 27 with a resinous material 29 in the manner taught in U.S. patentapplication Ser. No. 582,024, filed Sept. 26, 1966 now Patent No.3,374,452, by Winston R. Judd and assigned to the present assignee.

In the preferred embodiment, a 'brush holder assembly 28 is attached toa rotatable plate-like member 30, and a brush holder 32 urges aplurality of brush segments 34 into electrical contact with the track22. The plate-like member 30 has a plurality of ventilation openings 36therein, and a control knob 38 is provided for manually setting thelocation of the brush with respect to the brush track 22.

When the transformer is used to supply current to a load, the outputcurrent flows from the winding 20 through the brush segments 34, a brushsupporting plate 40 attached to the brush holder 32, the brush holder 32itself, and a conducting ring 42 as disclosed and claimed in U.S. PatentNo. 3,160,841 owned by the assignee of the present application. A brushmounted in a housing mem ber 44 bears against the conducting ring 42 andis connected to an output terminal 46 mounted on a terminal plate 48,the terminal plate 48 being in turn mounted on the base 27. A pluralityof terminals 50, 52, 54 and 56 on the terminal plate 48 enable a varietyof output voltages to be selected in order to provide a desired range ofvoltage for application to a load.

Referring now specifically to FIGURE 2, it will be seen that the shellhalves 16 and 18 comprising the shell 14 are substantially identical,with each including an annular outer peripheral surface 60 and 62respectively. On this surface there is mounted a carrier or backingstrip 65, the backing strip being disposed circumferentially around atleast a portion of the shell 14. The backing strip 65 used in theexemplification is a glass fibre strip with polyester resin initially ina semicured or B state impregnated therein. However, it will beappreciated that other suitable interstitial insulative materials may beused as the backing strip such as for example asbestos, cotton, silk,rayon, nylon, and the like formed with interstices for containing theresin. A principal requirement for the backing strip is that it beelectrically insulative and be capable of being impregnated and therebycarry the bonding material which in the exemplification as mentionedabove was a polyester resin. Such backing strips are commerciallyavailable, and one particular such strip is produced under'the trademarkFusa-Fab by the General Electric Company Other suitable thermosettingresins may be used, of course, instead of the polyester resins, such asthe acrylic resins, epoxy resins, ethylene propylene resins,polyurethanes, and oleoresins.

The winding 2 0;is, ofcourse, composed of a plurality of turns, eachindicated by reference numeral 70, and as the turns are disposedaroundthe shell. 14, a portion 72 of each of the turns 70, overlies thebacking strip 65 in the area of the brush track 22. Thus, as will beseen in FIGURE 2 in particular, the brush track 22 provided by thepresent invention. is unusually rigid and stable inasmuch as the turnportions 72 are. structurally bonded to the backing strip 65, and in theexemplification are in, fact partially embedded in the backing strip;However, it will be appreciated that it is not necessary for theturnportions 72 to be. embedded in. the strip but that all tur'n's merelymust be in adhesive or. bonding contact with the backing strip 65 inorder to provide the stable brush track provided by thepresentinvention. Also aswillbe seen in FIGURE .2, each turn portion 72has a segmen t removed, with the segments including at least the Wireinsulation and preferably a portion of the turns themselves in order toprovide a flat electrical contact surface for the brush track 22.

Referring now to FIGURES 3-7 inclusive, we will describe the preferredmethod of fabricating or producing the improved adjustable transformershown in FIG- URES 1 and 2. Referring initially to FIGURE 3, it will beseen that the shell halves 16. and 18 are assembled on the core 12,thereby substantially completely enclosing the core 12. While We haveshown, in the exemplification, the use of the shell halves 16 and 18 forinsulating the core, it will be appreciated that other suitableinsulating means might be used such as, for example, layer woundinsulation of various types or the like. In addition, it will beappreciated that an insulation layer may be provided on the core 12 bymeans of a carrier strip such as the strip 65 which could be disposed onthe core 12 between the core 12 and the winding 20. With the shellhalves 16 and 18 assembled on the core 12, the backing or carrier strip65 is disposed in substantial wrapping engagement about the peripheralsurfaces 60 and 62 of the shell halves, substantially encircling theshell halves. The strip 65 is retained in its position on the shellhalves by a suitable retaining means such as adhesive strips or thelike. With the backing strip 65 disposed on the shell 14, and With thepolyester resin impregnated therein being in a semicured, tacky state,the toroidal winding 20 is placed on the shell by winding a plurality ofadjacent turns 70 thereon, with portions 72 of the turns overlying thebacking strip 65.

In order to produce a stable, flat brush track 22 on the core 12, asshown in FIGURE 4, the core with the shell 14 and toroidal winding 20produced thereon is disposed on a rotatable table 80 having anupstanding mandrel or mounting post 82 thereon. The table 80 is turnedby a suitable means such as the worm gear 84 which meshes with a gear 86carried by the table 80 in order to present all the turn portions 72 toa hammering or pressing device 87 located adjacent the table 80. As theturn portions 72 are moved past the hammering or pressing device 87, thedevice is rapidly vibrated in order to straighten the turn portions 72and to press them against the backing strip 65. If desired, thehammering device 87 may be driven with sufiicient force to embedportions of the turn portions 72 into the backing strip 65. Thehammering device used in actual practice is a pneumatic hammer thatvibrates at a relatively high rate.

In the exemplified process, when all the turn portions 72 comprising thebrush track 22 have been subjected to the hammering or pressingoperation, the core and coil is dipped into a varnish tank 88 as shownin FIGURE 5 in order to fill the spaces left between the adjacent turnswith varnish. A varnish filler 90 is maintained in the tank 88 and thecore and coil is dipped into the varnish filler 90 by means of asupporting arrangement 92, with the core and coil being left in the tankfor approximately seconds. When the dipped core and coil is removed fromthe tank 88 .it is taken to an oven 94 and placed on a table 96 thereinin order to cure the varnish as well as to cure the semicured polyesterresin carried by the strip 65. It will be seen in FIGURE 6 that the oven94 is a generally conventional industrial type oven such as the typeusing electric resistance heating units 98 for heating purposes. It willbe understood, however, that any suitable heating means may be used tocure the varnish and the polyester resin. In actual practice, thevarnish is cured by permitting the core and coil assembly to remain inthe oven for approximately six hours at approximately 120 degreescentigrade. However, the polyester resin will cure in a much shortertime; for example, one type of polyester resin that we have used willcure to a tough, rigid mass in approximately one to one and a half hoursat 125 degrees centigrade. It will be appreciated that polyester resinsmay be cured by heat alone, or by a combination of heat and pressure.Therefore, if desired, the step of applying varnish to the assembly maybe omitted, and the polyester resin cured to structurally bond the turnportions 72 to the backing strip 65, and the backing strip 65 to theshell 14.

When the polyester resin has been cured and the turn portions 72structurally bonded to the backing strip 65, the core and coil is placedon a rotatable mandrel 100 at a grinding station 101 and the core andcoil rotated by shaft 102. A grinding head 104 carried on rotatableshaft 106 is actuated to grind off or remove outer segments of all ofthe turn portions 72 in the area of the brush track 22 to produce aflattened surface. At this time, the core and coil is assembled with thebase plate 27, and the remaining components of the transformer 10 areassembled thereon to produce the final transformer unit 10. However, ifdesired, the core and coil may be subjected to additional processingsteps, e.g., (1) an additional varnish dip and cure after the first cure(to insure a water seal, as certain types of wire enamels are sensitiveto attack by water); and/or (2) plating the coil turns at the brushtrack 22 with gold or other suitable electrical contacting surfaceplatings to provide good electrical contact w 1th the brush segments 34and to prevent excessive wearlng of the turns 70. I

In the actual reduction to practice of our invention, 1n certain unitsall of the above described steps are carried out, while on other unitssome of the steps are omitted. In practice, we use a backing strip thatis approximately 0.03 of an inch in thickness, but we may, of course,use thicker or thinner strips so long as they carry a resin composition.

From the aforegoing description, it will be appreciated that we haveprovided an improved variable transformer of the adjustable brush typehaving firmly bonded turns that are not readily displaced duringmanufacture or operation. While we have illustrated and described aparticular exemplification of the invention, it will be apparent thatmany modifications will occur to those skilled in the art. Thus, thisinvention is not intended to be limited to the particular embodimentshown and it is intended by the appended claims to cover allmodifications that come within the true spirit and scope of theinvention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. An adjustable sliding brush transformer comprising: an annular coreof magnetic material; a shell of insulating material disposed on saidannular core; a backing strip of interstitial insulating materialdisposed circumferentially around at least a portion of said shell, saidbacking strip having a cured thermosetting resin impregnated therein; awinding of successively disposed adjacent wire turns wound on said shelland overlying said backing strip, .said winding having a flat electricalcontact surface portion, and said cured thermosetting resin in saidbacking strip bonding the electrical contact surface portion to saidshell; and electrical brush means for sliding electrical contact withsaid flat electrical contact surface.

2. The adjustable sliding brush transformer of claim 1 wherein saidbacking strip comprises a glass fibre strip wrappingly engaging an outersurface of said shell and wherein said thermosetting resin is apolyester that is initially in a semicured state prior to Winding ofsaid wire turns on said shell.

3. In an adjustable sliding brush transformer having an annular core ofmagnetic material, an insulating means covering substantially saidmagnetic core, a toroidal winding including Winding turns wound on saidinsulating means and magnetic core, and a movable brush means for makingelectrical contact with said toroidal winding, the improvementcomprising: an electrical brush track formed by a contact surfaceportion of said winding, a permeable backing strip disposed between theinsulating means and said winding adjacent to said contact surfaceportion, said permeable backing strip having cured thermosetting resinimpregnated therein, said contact surface portion being 7 a partiallyembedded in said backing strip, and said backing strip bonding saidcontact surface portion to said insulating means.

4. The adjustable sliding brush transformer of claim 3 wherein saidthermosetting resin impregnated in said backing strip is in a semicuredstate when said toroidal winding is placed over said insulating means onsaid magnetic core.

5. A method of producing a variable transformer having an annular coreof magnetic material, a toroidal winding carried on the core andelectrical brush means disposed for sliding contact with the toroidalwinding, the methodcomprising the steps of: assembling on the annularcore at least one insulating member having an outer annular surface,applying a backing strip impregnated with a thermosetting resin on theouter annular surface of said insulating member in substantial wrappingengagement therewith, said thermosetting resin being in a semicuredstate; placing the toroidal winding on said insulating member andannular core by winding a plurality of adjacent turns thereon withportions of the turns overlying the backing strip; curing thethermosetting resin;

and forming a contact surface on said toroidal winding to provide abrush track.

6. The method of claim 5 including the step of embedding the turnportions at least partially into said backing strip by applying acompressive force during winding thereby to force the turn portionsradially inwardly into the backing strip.

References Cited UNITED STATES PATENTS 1,402,587 1/1922 Evershed338-474- XR 2,147,824 2/1939 Webb 336205 XR 2,246,159 6/ 1941 Work etal. r 336205 XR 2,949,592 8/1960 Smiley 336-448 3,316,517 4/1967 Ellin33148 XR 3,339,162 9/1967 Burnsteel et al 336205' 3,396,356 8/1968Whipple 336-205 XR LEWIS H. MYERS, Primary Examiner 20 T. .T. KOZMA,Assistant Examiner US. Cl. X.R.

